Transgenic technology, involving either the addition or deletion of a gene from the mammalian genome, has proven to be one of the most valuable tools for the study of genetics. Transgenic animals have been crucial to the knowledge of human genetic diseases, developmental programming, and genetic interactions. Transgenesis has also been used for the improvement of livestock and for the production of pharmaceuticals. The generation of transgenic animals by microinjection of the transgene into fertilized eggs is reproducible and there has been no necessity to improve what is already working. However, a significant drawback of this transgenic technique is that it relies on a plentiful supply of fertilized eggs. In larger animals such as primates, pigs, and cows, the cost of producing 500 or more eggs (at 10-20 eggs or less/animal) is prohibitively expensive. Yet there are many instances when transgenic primates would be a more appropriate model for human diseases and for testing therapies. Large domestic animals would be more effective for the commercial production of therapeutic drugs and for xenotransplantation. Since the testis contains an abundance of germ cells [e.g., 0.5 x 10(8)/mouse testis], the applicants propose to generate transgenic animals using male germ cells. Transgenic animals will be generated by two different methods: 1) direct delivery of genes to male germ cells in vivo using viral vectors; and 2) germ cells will be harvested from the testis, transfected with DNA in vitro, and stable cells transplanted into either chemically-treated or irradiated testes that contain no endogenous germ cells. The transfected germ cells would repopulate the testis and the animals would produce live transgenic offspring by natural mating. Transgenic animals produced by either of these two techniques will likely be efficient, cost-effective, and will require far fewer animals.